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Mass. health officials announce additional human cases of West Nile virus

The risk of West Nile virus is now high in 47 communities across Suffolk, Middlesex, Essex, and Worcester counties.

Mosquitoes are trapped and tested for viruses, included West Nile Virus. Ben Garver

September 27, 2025

2 minutes to read

State health officials on Friday announced two new human cases of West Nile virus in Massachusetts this year.

According to the Massachusetts Department of Public Health (DPH), one case involves a woman in her 60s who was exposed to the mosquito-borne illness in Middlesex County. The second case involves a man in his 50s who was exposed in Suffolk County. These bring the total number of confirmed human cases of West Nile Virus this season to eight.

On Friday, public health officials elevated the West Nile virus risk to high in Arlington, Chelsea, Medford, and Waltham, increasing the total number of high-risk communities across Suffolk, Middlesex, Essex, and Worcester counties to 47.

An additional 210 cities and towns in Barnstable, Berkshire, Bristol, Essex, Franklin, Hampden, Hampshire, Middlesex, Norfolk, Plymouth, Suffolk, and Worcester counties are currently classified at a moderate risk level.

“We are almost out of the peak season for the spread of West Nile virus to people in Massachusetts,” said Public Health Commissioner Robbie Goldstein, MD, PhD. “Over the last 10 years, 80 percent of people became sick in August or September, with 12 percent developing illness after Oct. 1. But because some risk will continue until the first hard frost, we are encouraging everyone to continue to take the necessary steps to protect themselves from mosquito bites by using mosquito repellent and wearing long sleeves and pants to reduce exposed skin.”

Most people infected with West Nile virus do not develop any symptoms, according to the Centers for Disease Control (CDC). However, about one in five will come down with severe illness with high fever, headache, and disorientation. Some patients may also experience coma, tremors, convulsions, muscle weakness, vision loss, and paralysis.

DPH officials announced the first West Nile Virus-positive mosquitoes of the season on June 17. Since then, 445 mosquito samples have tested positive for the virus in Barnstable, Berkshire, Bristol, Dukes, Essex, Franklin, Hampden, Middlesex, Norfolk, Plymouth, Suffolk, and Worcester counties. There has also been one animal case in a goat, identified in July.

Another mosquito-borne illness, Eastern equine encephalitis (EEE), was also detected in Massachusetts mosquitoes this year. To date, 23 mosquito samples have tested positive for EEE, though no human or animal cases have been reported.

EEE is rarer than West Nile virus but causes more serious illness. According to the CDC, about 30 percent of people with severe cases die. Those who survive are often left with long-term neurological complications. Symptoms of EEE infection include fever, headache, vomiting, diarrhea, seizures, changes in behavior, and excessive drowsiness.

To lower the risk of infection, health officials are urging people to avoid mosquito bites by applying insect repellent when outdoors — particularly repellent with an EPA-registered ingredient, like DEET.

People should also avoid outdoor activities during peak mosquito hours, which run from dusk to dawn, and wear long sleeves, pants, and socks if outside during that time.

Other safety measures include installing or repairing window and door screens and draining standing water from buckets, tires, and wading pools — especially after heavy rains.

For horse owners, experts say water troughs are a haven for mosquito breeding and should be flushed out to reduce mosquitoes near paddock areas. At night, horses should be kept in indoor stalls to reduce the risk of being bitten.

The Massachusetts Arbovirus Update page shares new cases of West Nile virus and EEE daily. DPH officials urge residents to check in regularly to learn the risk levels in their community and around the state.